For n identical resistors of resistance R: Series combination: S = nR Parallel combination: P = R/n Given 5 = nP, substituting P = R/n: 5 = n (R/n) ⇒ 5 = R, so n = 4. Answer: (d) 4.

Total resistance Rₜ = 21Ω + 4Ω = 25Ω. Using V = IR = 0.2 × 25 = 5V. Power P = VI = 5 × 0.2 = 1W = 1 J/s. Answer: (d) 1 J/s.

Let each cell have emf E and internal resistance r. For aiding: (12E + 2E) / R = 3A → 14E/R = 3. For opposing: (12E – 2E) / R = 2A → 10E/R = 2. Solving, x = 2 ...

For two parallel 1.5V cells, total external resistance is 10Ω. Using V = E – Ir and solving with V = 1.2V, we get r = 5Ω. Answer: (c) 5Ω.

The heat produced in a conductor is proportional to the square of the current (H ∝ I²). If the current is tripled, the heat increases by 3² = 9 times. Thus, the temperature rise will be 4°C × 9 = ...